The specification refers to and describes content of U.S. Pat. Nos. 5,520,679, 6,339,278, 6,339,470 and 6,342,751. However, neither the disclosures in those US patents nor the description contained herein of content of those US patents is to be taken as forming part of the common general knowledge solely by virtue of the inclusion herein of the reference to and description of content of those US patents. Furthermore, this specification describes aspects of prior art optical scanning systems. However, neither such aspects of prior art optical scanning systems nor the description contained herein of such aspects of prior art optical scanning systems is to be taken as forming part of the common general knowledge solely by virtue of the inclusion herein of reference to and description of such aspects of prior art optical scanning systems.
A wide range of lasers are suitable for the above applications, including: excimer lasers, Nd:YAG, Nd:YLF, Er:YAG, Nd:KGW, Carbon Monoxide, and Carbon Dioxide lasers. The wavelengths produced by these lasers range from deep in the ultra-violet (UV) to long infra-red (IR) wavelengths.
A feature that is often common among the use of these lasers for material processing is the need to move the laser beam relative to the material surface being processed. When the material is not deliberately being moved and the laser beam is being directed to carry out the processing, the movement of the laser beam is often performed by galvanometer or motor driven mirrors and lenses.
In the field of treating refractive errors by laser ablation, J. T. Lin (U.S. Pat. No. 5,520,679) proposed using galvanometer scanners to control a low energy laser beam into an overlapping pattern of adjacent pulses to produce the desired change in the corneal surface. U.S. Pat. No. 5,520,679 states that this allows a smaller, lower cost laser to be used for this procedure. U.S. Pat. No. 5,520,679 also states other advantages, including a reduced need for a homogenous beam and better flexibility in design of the treatment.
Galvanometer scanning excimer lasers are currently one of the most common means for correcting refractive errors using the LASIK surgical procedure.
Although galvanometer scanners have been very successful in scanning lasers for reshaping corneal tissue and a large range of other applications, they do have some disadvantages. They have a trade-off between the size and weight of the mirror being tilted and the speed by which the galvanometer can adjust its position. Sometimes this results in mirrors that are not large enough for the optical system or using mirrors that are too thin to maintain their required flatness during the scanning process. Galvanometer scanners also have limited accuracy when the desired scan angle is small (less than 3 degrees).
The galvanometer scanners used in refractive lasers generally work well at the pulse repetition rates currently used, i.e. 200 Hz or below. However, this assumes that the eye is not moving. Tracking the eye has now become an important part of producing good results for refractive surgery. Between each pulse the position of the eye is measured and then the scanner position adjusted to compensate for any eye movements before the eye moves again. This means that the scanner must be capable of moving much faster than when the laser was operating without an eye tracker. These faster response requirements from the scanner go beyond the response capabilities of galvanometers. This becomes even more of a problem when the demands of customised surgery require smaller spot sizes to ablate with higher precision and subsequently much higher pulse repetition rates. Galvanometric scanners would not have adequate response for such a laser system.
A problem that sometimes occurs in galvanometric scanners if the eye moves slightly up and down, getting closer or further away from the laser, is that each pulse may not hit the eye in the correct position. Because of the scan angle, if the eye is too close to the laser system then the pulses over-lap more than intended and the total area exposed to the laser is less than intended. If the eye is too far from the laser the opposite occurs. In either case, the result of the surgery is degraded.
An alternative drive mechanism to a galvanometer drive mechanism is a piezoelectric drive.
Piezoelectric drives have the advantages of having potentially infinite precision and are capable of generating extremely high forces, so could drive a large mirror very fast. However, piezoelectric drives also have a number of significant disadvantages, and although they have been used to scan laser beams, they have not been generally accepted for this type of application because of these disadvantages.
The main disadvantage of piezoelectric drive systems is their very limited range of movement. They are therefore not considered to be a potential means of scanning in applications currently performed by galvanometer scanners. One method that has been used to amplify the range of piezoelectric scanning is to have the piezoelectric crystals push or pull on the end of a metal plate. The metal plate bends and deflects a mirror further than the same piezo would move the mirror if applied behind the edge of the mirror. A device based on this technique is described by Takeuchi et al in U.S. Pat. No. 6,342,751. However, this type of technique creates a non-linear beam deflection and loses much of the potential accuracy of a piezoelectric drive mechanism, and still has a much smaller range of scanning than galvanometer scanners. These types of techniques also suffer from reduced response time, stiffness and have a significantly smaller force/load capability.
The second significant problem with piezoelectric drive systems, or actuators, is that they have significant hysteresis. This is normally in the order of 10% to 15% of the range of the movement. This hysteresis is another key reason why piezoelectric driven scanners are currently not used in applications requiring fast complex scan pattens, such as laser systems for refractive surgery. This hysteresis can be corrected by operating the piezoelectric system in a closed loop fashion. This requires a sensor to measure the movement of the system and then a controller that adjusts the voltage to the piezoelectric actuator so that it moves to the desired position. The problem with this is that it significantly reduces the response of the system, and its accuracy is reduced to the accuracy of the sensor. In an application in which tolerances are critical, such as refractive surgery, the hysteresis induced error can be so large that the piezoelectric signal and position sensor signal cannot be compared to check the system is operating correctly. So to achieve a redundant check of scanning performance a second position sensor would need to be used.
Papademetriou, et al in U.S. Pat. No. 6,339,470 describes means for scanning lasers across optical fibres. This US patent also describes use of a piezoelectric stack to adjust the angle of a mirror. However, this description complains of the lack of range of such a scanning mechanism as special effort is required to scan the laser across the entrance of a single optical fibre. The main scanning mechanism used in the device described in this US patent relies on acousto-optic deflection of the laser beam, where that scan range must cover more than one optic fibre (which is smaller than the range across an eye). Acousto-optic scanners are relatively complex, have high optical losses and are not suitable for many of the wavelengths used for material processing applications.
The background description in U.S. Pat. No. 6,339,278 (Shinohara, et al) describes conventional inclination optical scanners and lists galvanometers, stepper motors and other mechanisms as examples but not piezoelectric mechanisms. However, the invention described in this US patent does use a piezoelectric device, but it is used as a mechanical oscillator to drive an ultrasonic motor that deflects the laser beam.